To increase the power of internal combustion engines, exhaust gas turbochargers are used. Due to the large operating range of internal combustion engines in passenger vehicles, the exhaust gas turbocharger must be regulated in order to achieve a set boost pressure. To this end, in a multistage supercharging system (i.e., one with exhaust gas turbochargers connected in series), the fresh air is compressed first in a low-pressure compressor of a first exhaust gas turbocharger and then in a high-pressure compressor of a second exhaust gas turbocharger. When large volumes of fresh air are present, the choke limit of the high-pressure compressor is exceeded. To keep the high-pressure compressor from functioning as a choke in this case, some of the fresh air can be diverted around the high-pressure compressor through a compressor bypass. When the volume of fresh air is below the choke limit of the high-pressure compressor, the compressor bypass is closed.
To keep the pressure build-up in the exhaust gas turbocharger from not lagging when the temperature of the exhaust gas is low and the volume of exhaust gas is very small, as is the case at low rpm, exhaust gas turbochargers of the kind currently used in internal combustion engines have a very low intrinsic mass and therefore respond even at low exhaust flow rates. The power limits of the exhaust gas turbocharger can be broadened for example by regulated two-stage supercharging, as known from Bosch, Kraftfahrttechnisches Taschenbuch [Automotive Handbook], 23rd Edition, Vieweg, 1999, pages 445-446. In regulated two-stage supercharging, two exhaust gas turbochargers of different sizes are connected in series. The stream of exhaust gas first flows into an exhaust manifold. From there, the exhaust gas stream is expanded via a high-pressure turbine. When large volumes of exhaust are present, as at high rpm, a portion of the mass flow of the exhaust gas can be diverted around the high-pressure turbine through a bypass. The entire exhaust gas mass flow is then utilized by a low-pressure turbine downstream of the high-pressure turbine. The mass flow of aspirated fresh air is first precompressed by a low-pressure stage and then compressed further in the high-pressure stage. Ideally, the fresh air mass flow is intercooled between the low-pressure stage and the high-pressure stage.
At low engine rpm, i.e., low exhaust gas mass flow rates, the bypass circumventing the high-pressure turbine remains completely closed and the entire exhaust gas mass flow is expanded via the high-pressure turbine. This produces a very rapid and high build-up of boost pressure. As the rpm increases, the expansion work is continuously shifted to the low-pressure turbine by virtue of a corresponding increase in the cross section of the bypass. Thus, regulated two-stage supercharging permits infinitely variable adjustment to engine demands on the turbine and compressor side. Due to the decreasing flow of exhaust gas through the high-pressure turbine, the compressor power of the high-pressure compressor also decreases. When large fresh air mass flows are present, the compression is done by the low-pressure compressor alone. Fresh air does flow through the high-pressure compressor, but the pressure before and after the high-pressure compressor is the same. As soon as the choke limit of the high-pressure compressor is exceeded, that is, once the stream of fresh air flowing through the high-pressure compressor exceeds the volume flow that the high-pressure compressor can handle without pressure loss, the high-pressure compressor acts as a choke and the pressure of the fresh air decreases as it flows through the high-pressure compressor. To keep the choke limit from being exceeded, when fresh air mass flow rates are high, a portion of the fresh air is diverted around the high-pressure compressor through a compressor bypass. The compressor bypass contains a valve that closes or opens the bypass. This valve is currently controlled by means of an external control unit.
A sequence valve for sequential supercharging using two exhaust gas turbochargers is known from ATZ Automobiltechnische Zeitschrift 88 (1986), page 268. At low rpm, the sequence valve initially causes the fresh air to bypass one of the two compressors. The second compressor is not tied in until higher rotational speeds are reached. For this purpose, the bypass is made to accommodate a displacement body, the upstream and downstream sides of which are both subjected to a pressure force in the closed state. As long as the pressure force on the downstream side is greater than that on the upstream side, the valve is closed. As soon as the pressure on the upstream and downstream sides is equal, the valve opens. Since the entire displacement body is moved each time, a relatively large mass must be moved in order to open and close the bypass. This makes for relatively slow opening of the valve.
An internal combustion engine provided with supercharging in a high-pressure turbine and a low-pressure stage that is larger than the high-pressure stage is disclosed in DE A [Published German Patent Application] 195 14 572. To obtain the most lag-free supercharging possible, the high-pressure turbine and the low-pressure turbine are initially connected in series. At about 50 to 60% of the rated rpm, the exhaust gas is diverted completely around the high-pressure turbine through a bypass. This consequently simultaneously shuts off the high-pressure compressor, which is driven by the high-pressure turbine and is connected in series to a low-pressure compressor driven by the low-pressure turbine. In this case, the high-pressure compressor is circumvented via a boost-air line that contains a check valve to prevent boost air from flowing back through the boost-air line while the high-pressure compressor is operating. However, DE A 195 14 572 gives no indication of whether the check valve is self-actuating or is controlled from the outside. Nor does the document disclose the differential pressure at which the check valve closes the boost air line.